1. Field of the Invention
The present invention relates to a viewfinder which has an indicating frame in a field of view, for example a viewfinder which is provided in a camera, separately from a photographic optical system of the camera.
2. Discussion of the Background Information
One example of a conventional viewfinder has been used is referred to as a real image type finder. In a real image finder an observer observes a real image of an object formed by the objective optical system of the finder through the eyepiece optical system.
In such a real image type viewfinder, generally, either an indicating frame, such as a field frame representing an area to be photographed through a photographing optical system of a camera, or a focusing frame (i.e., an objective distance measuring frame) representing an area for measuring the distance of the object from the camera by an object distance measuring device, is drawn on one surface of an indicating plate (or a finder screen). The indicating frame is disposed at the same position as an image, formed by the optical image system, of an object which is located a predetermined distance away from the camera. The predetermined distance will be referred to as a reference objective distance hereinafter. The indicating screen comprises a plane-parallel transparent plate made of glass, plastic, or similar material, i.e., a substantially flat plate having parallel planar surfaces. Thus, this type of viewfinder is arranged so that an observer is able to observe the image of the indicating frame with substantially the same visibility as the objective image is seen through the eyepiece lens of the viewfinder.
Another example of a conventional viewfinder which is used is an inverted Galilean type Albada finder. The optical system of this type of finder comprises an objective lens, which has a negative refractive power so as to form a virtual image of an object, and an eyepiece lens, which has positive refractive power so as to magnify the virtual image
In this type of viewfinder, an indicating plate which comprises a plane-parallel transparent plate is disposed between the objective lens and the eyepiece lens. The object side surface of the plate, i.e., the surface of the plate closest to the object, has an indicating frame drawn thereon. This frame is a bright frame formed by vapor deposition of metal. The surface of the objective lens which is adjacent to the eyepiece is formed with a negative curvature and is coated with metal so as to function as half-mirror.
The position of the indicating frame is arranged so that the position of the image, which is formed by a reflection of the indicating frame on the surface of the objective lens, coincides with the position of the virtual image of the object at said reference objective distance, which image is formed by the objective lens.
In such conventional viewfinder optical systems, however, because the indicating frame is drawn on the surface of a plane-parallel transparent plate, and because the surface of the indicating frame plate which is drawn on the indicating frame is exposed to air, it is possible that foreign matter, such as dust, cut chips produced during the manufacturing process of the finder, or drips of water, will stick to the surface of the plate. In this case, the indicating frame and the surface on which the frame is drawn are located at the same optical axial position, so that once even small foreign matter sticks to the surface having the indicating frame thereon, the foreign matter and the indicating frame will be observed in the same condition of visibility through the eyepiece lens Further, foreign matter on the plate is magnified by the eyepiece, even if it is small. Therefore, one problem which arises in these viewfinders is that foreign matter is conspicuously seen in the field of the viewfinder. Further, because it is difficult to remove such foreign matter during camera assembly, to do so would unduly slow the production process and reduce the rate of camera production
Further, in such conventional viewfinder optical systems, the indicating frame plate is thin and the indicating frame is merely drawn on, or attached to, the object side surface of the plate. However, the imaging position of an image of an object, which is formed by the objective lens, axially varies according to the objective distance of the object, so that when an observer observes the image of the object formed by the objective lens with the eyepiece lens, the observer observes the image by varying the focus of the eye of the observer according to the axial position of the image, which varies as described above. In other words, the observer observes the virtual image which is formed by the eyepiece lens and which is based upon the image of the object which is formed by the objective lens. This circumstance is discussed hereinafter as well. Therefore, when the observed object is at the objective distance where the position of the image of the object formed by the objective lens coincides with the position of the indicating frame or its image, i.e., the reference objective distance, the observer can observe the object and the indicating frame clearly at the same time. However, usually the image of the object formed by the objective lens does not coincide with the position of the indicating frame or its image, and an observer will therefore not be able to observe them clearly at the same time, so that the observer must alternately move and adjust the focus of the eye of the observer for the object image and for the indication frame or the image of the frame. As a result, another problem arises in that the observer will find it difficult to observe both the object image and the indication frame or the frame image, and will accordingly tend to become fatigued.